November 24. Mammals

Mammals diversified during the Cretaceous, so that by the end of the period, the two major modern groups, the placentals and the marsupials, were well established. Placental mammals include most modern types including rodents, primates – most every kind of mammal other than marsupials and the egg-laying monotremes, which are platypuses and spiny anteaters.

The mutituberculates were rodent-like mammals that got started during the Jurassic, thrived in the Cretaceous and survived the end-Cretaceous extinction, only to disappear in the Oligocene about 30 million years ago. Their 120-million-year run is the longest of any mammal lineage, and their diversity is reflected in at least 200 different species. Their name comes from their teeth, which have rows of little points, or tubercules. Although they occupied rodent-like niches, including burrows and trees, and were superficially much like squirrels and rats and other rodents, they are classed taxonomically in their own order, and they have no modern descendents. It looks like the true rodents displaced them in the early part of the Cenozoic Era, which we’ll talk about next month.   

Brian Switek has a post this week on a new Cretaceous mammal discovery, Vintana, a muskrat-like critter that lived in Madagascar a few million years after that island finally became separated from larger land masses. Madagascar had pulled away from Africa while it was still attached to India during the latter part of the Early Cretaceous epoch. India and Madagascar separated around 90 million years ago, and Vintana dates to about 70 million years ago, so it might be a reflection of evolution in the relative isolation of a small island continent. Check the Laelaps blog at phenomena.nationalgeogeographic.com for more information on this and many other fossil topics. 

Repenomamus with dinosaur bones in stomach –
photo by David Wong, used under Creative Commons license.  

The common view of Cretaceous mammals living in fear of dinosaur predators, creeping through the underbrush at night, was turned topsy-turvy by the discovery of a meter-long mammal in China, called Repenomamus. One specimen was found with fragments of a juvenile herbivorous dinosaur in its stomach. We don’t know if it scavenged, or chased small dinosaurs down, or maybe it preyed on the young. But it did eat dinosaurs. At a meter long, three feet or so, Repenomamus is the largest known Mesozoic mammal. Most Cretaceous mammals were around 3 to 10 inches long. Many are known only from a single tooth or a few bones, so they are not really very well known. Some spectacular exceptions exist, including an early Cretaceous mammal fossil from China in which the 4-inch-long animal’s fur can be clearly seen.

We’re coming up on the end of the Cretaceous in a week, and the last day of November will be devoted to the extinction event at the end of the period. I wanted to mention today some ideas that addresses the question of how mammals and birds survived the extinction event when, so far as we know, no dinosaurs did. One popular idea has been that birds and mammals in general were smaller in size than most dinosaurs, although there were a few exceptions at times. Smaller size would have allowed animals to maintain and regulate body temperature more easily, in the event of climatic extremes such as those that might have accompanied the end-Cretaceous extinction. And many mammals were burrowers, which would also afford protections from cold or other climate changes.

Mammal and bird brains might have become complex enough by the end of the Cretaceous, and more complex than the non-avian dinosaurs, so that as individuals and as groups they were simply more adaptable to the challenging conditions during the extinction event. Some mammals and birds did die off at the end of the Cretaceous, so it was most definitely not a case of all mammals and birds good, all dinosaurs bad.

The idea of long-term hibernation, say something like three-quarters of the year, has been offered as a way some mammals might have made it through a nuclear winter or other ecological catastrophe at the end of the Cretaceous. That might work for some, but certainly not for all mammals and birds. When it’s all said and done, a lot of the thought about mammal and bird survival at the end Cretaceous is largely common sense and reason – and nothing is certain yet to make it clear why they did survive.

—Richard I. Gibson

Repenomamus with dinosaur bones in stomach – photo by David Wong, used under Creative Commons license.  

Hibernation?

Surviving the extinction 

Laelaps blog

Mammals ate dinosaurs